This invention relates generally to a control system for a fluid heated steam generator of the once-through or low-recirculation-rate type. The invention is of particular significance to steam generators that are heated by the thermal output of nuclear reactors where, for example, a liquid metal is the heating fluid.
The different heat transfer regions that may be present within a steam generator are normally identified by the terms "sub-cooled", "nucleate boiling", "film-boiling", and "superheat". The "sub-cooled" region is that region in which the water is below the saturation temperature at the pressure extent within the tube and the heat-transfer rate is somewhat less than in the "nucleate-boiling" region. The "nucleate-boiling" region is that region in which ebullition takes place at a solid-liquid interface and the tube inner-surface temperature approaches the water saturation temperature. The "film-boiling" region is that region in which a film of super-heated steam forms over part or all of the heating surface and the heat-transfer rate is greatly reduced compared to that in the "nucleate-boiling" region. The "superheat" region is that region in which the steam quality is 100% and the bulk steam temperature is above the saturation temperature. When the location of the nucleate-boiling region is uncontrolled, it shifts back and forth along the length of the steam generator. This is extremely undesirable as the generator tubes are subject to repeated thermal cycling causing creep-fatigue damage.
Control of once-through or low-recirculation-rate liquid-metal-heated steam generators has been accomplished by processes that use measurements of water-side parameters such as steam flow, steam temperature, feedwater flow, and feedwater temperature, as well as various heat-balance calculations. These control systems are capable of maintaining the outlet steam pressure and temperature within reasonable bounds, while preventing gross instability in the nucleate boiling region within the steam generator. However, these control systems provided little or no control over the location of the nucleate boiling region within the steam generator. Furthermore, they have limited ability to control the location of the nucleate boiling region during changes in load or upsets in the heating-fluid or water systems. Direct measurement of tube wall temperatures is impracticable in commercial units because of the difficulty in maintaining temperature sensors and the large number of measurements that would be required to obtain a sample representative of the average temperature of the tubes at specific elevation. Concern for controllability and stability of the location of the nucleate-boiling region in liquid-metal heated steam generators of the once-through type has caused some plant designers to choose less efficient systems containing natural-circulation evaporators, steam drums, and separate superheaters.
Therefore, it is an object of the present invention to provide a control system for fluid heated steam generators.
It is another object of the present invention to provide a method of controlling the location of the nucleate-boiling region within a fluid heated steam generator.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.